There have been an increasing number of new threads which are asking pretty similar questions about ActiveSync problems, so here's the thread to compile the guides & links. If anybody else have found other useful links/threads not mentioned here, please contribute to the thread.

Firstly, here's a great site that has a huge list of common ActiveSync errors and how to counter them:
ActiveSync Troubleshoot Guide

Here's the official page for Microsoft ActiveSync, and it also has several FAQs and guides there:
Microsoft ActiveSync Official Page

If your PC is running on Windows Vista, you have to use Windows Mobile Device Center instead of ActiveSync.

Here's another useful official guide:
ActiveSync 4.x USB Connection Troubleshooting Guide

Here are some links for downloading ActiveSync:
ActiveSync 4.5
ActiveSync 4.2 (No longer available)
ActiveSync 4.1 (No longer available)
ActiveSync 3.8 (No longer available)

Here are some useful threads regarding ActiveSync:
Changing ActiveSync Partnership Name
How to delete existing/old ActiveSync Partnership?
Tech Guide: ActiveSync Via Bluetooth
Bluetooth ActiveSync Guide 1
Bluetooth ActiveSync Guide 2
ActiveSync via Wi-Fi (for ActiveSync 3.x)
First Time Installing ActiveSync?
Firewall Settings for Connecting to a WM5 Device via USB Sync

Add-On ActiveSync Components:
MightySync - Sync any location on Pocket PC with any location on PC
ManyPartners - Break the ActiveSync Partnership limit. Sync with more than 2 PCs.
MobSync - Sync folders other than "My Documents"
SMS Cool! - Send SMS from the PC via the connected PPC-Phone
SyncExpress - Synchronise with Outlook Express on the PC.
Remote Display Control - Control Pocket PC from the PC (Refer to this thread for VGA mode)
Remote Keyboard - Input using keyboard from a PC via ActiveSync connection
Theme Sync - ActiveSync extension which rotates your themes.

Other Related Software:
ActiveSync Toggle - Enable/Disable ActiveSync. Disabling ActiveSync will remove the icon from system tray on PC.

Other Synchronisation Software:
PocketMac - Sync Pocket PC/Smartphone with PC on Mac OS
The Missing Sync - Sync Pocket PC/Smartphone with PC on Mac OS

Here's the standard procedure for checking ActiveSync if you have problem connecting your PPC to the PC for the first time:

1. Have you installed ActiveSync successfully? ActiveSync may take quite a long time to install, so please be patient.
2. Have you installed Microsoft Outlook?
3. Have you launched Microsoft Outlook at least once to go throught the basic settings?
4. Have you allowed ActiveSync program access in your firewall settings?
5. Have you allowed the network created by ActiveSync 4.x access (with WM5/WM6 device) in your firewall settings?
6. Do you have SP2 for WinXP if you are running ActiveSync 4.x?
7. Do you have ActiveSync 4.5 if you are synchronising with a WM6 device?

If you are unsure about point 4 & 5, you can disable your firewall and try to sync to check if it's a firewall setting issue. If you don't know what firewall is and you are running WinXP SP2, then firewall is enabled by default.

If this is the first time you are synchronising a WM5/WM6 device and did not have problems with devices running on older OS, this thread should be able to help. If the ActiveSync icon (for ActiveSync 4.x with WM5 device or v4.5 with WM6) is spinning forever but does not start synchronising, the same solution also apply.

For configuring firewall, this page should also be very helpful.

If you have experienced "Error 608" while trying to sync, these 2 threads may be useful references:
http://www.ppcsg.com/http://www.ppcsg.com/index.php?showtopic=62499
http://www.ppcsg.com/http://www.ppcsg.com/index.php?showtopic=86072

If you have problems with internet connection via ActiveSync passthrough, check the following:
1) Ensure you have successfully synchronised before. This is to check that firewall is not blocking it.
2) Try again after disconnect and re-connect.
3) Try again after soft reset and re-connect.
4) On PC, go to File=>Connection Settings in ActiveSync's menu and try changing "This computer is connected to:" to "The Internet".
6) On the device, go to "Connections" settings and go to the "Advanced" tab. Click on "Edit" and check that all the checkboxes in the "Proxy Settings" page are unticked. If it does not work, continue to next step.
7) Tap on "Select Networks" and try changing the first dropdown option to "My ISP".
8) If you have problems with GPRS connection because you changed the default connection to "My ISP", please refer to this post and this thread for the manual GPRS config guide.

Palm's Customer Support knowledge Library has updated their ActiveSync error messages when trying to sync a Windows Mobile device page, more Support code and Troubleshooting information included, check out the page if you do encounter any problem (touchwood!) when trying to sync your device.

Activesync Support code

I have a problem with ActiveSync and the error has become constant : Runtime error relating to the WCESMgr.exe

I have uninstalled and reinstalled ActiveSync several times including downloading latest installation file from Microsoft Website in case my saved version is corrupted.

Any specific advice as I can't seem to find a solution in the ActiveSync Guide? Tks

Did you uninstall and reboot first before you install and reboot again?

Yes, when I uninstall the ActiveSync from my laptop, I will restart the laptop before installing again.

Is the Mircosoft Outlook = Outlook Express?

I kept getting this error even tho my outlook express is out.

Nope, it's not Outlook Exporess. You need to purchase additional software in order to sync with Outlook Express.

Hi there,

Hope someone can help..
I have installed ActiveSync on my computer. And i have 2 different users registered on my PC. Is it possible to sync 2 different Windows mobile? i.e user A sync mobile A and user B sync mobile B. Want to find out first before trying as i don't want to take the risk of the 2 mobile devices data getting mixed together. Thanks in Advance.

As long as you login to the correct user profile on the PC before you do the sync, it should be ok.

Thanks Vicott. Will try it out. Can i ask 1 more question? I am going to update my Palm 750v to WM6. Will the update erase all my sms? Thanks again.

Yup therefore please back up your important data first before you attempt to hard reset your device.

thanks lmf, will look through the forum on how to back up sms. Cheers.

backup sms? I m sure u cn find 1. I used 1 for my D810 previously but couldn't recall the name.

Please keep to the topic. Thanks.

Anyone know how to prevent active sync from running whenever MapKing/GPS is running? I read it somewhere but not able to locate the thread anymore. Thanks!

You are can download the NOGPRS which allows you to toggle your GPRS connection ON and OFF from the thread HERE...

Hi,

I don't think the NO GPRS stop the active sync from running up with MAPKING running.

Thanks

i am using vista on my laptop, can i install Microsoft Outlook 2002 which i have in my collection..will it be compatible? my laptop is not pre-installed with Microsoft Ofiice n Microsoft outlook. :( just bought a samsung sgh-i780.

I used to be able to 'schedule' sync on Activesync 4.1 with my XDA O2ii, but since I upgrade to Activesync 4.5 & Asus P750, i cannot set the sync schedule, ie 'Schedule' tab cannot be activated, is that common?

hihi,
im having prob with active Sync fir my HTC touch diamond. help please..
thank u =)