Yes, there are all kinds of multiplayer games for the PPC that connect via Bluetooth or WiFi; multiplayer board games, multiplayer racing games, multiplayer flight simulators, sports, strategy, blah, blah, blah... you name it. Handmark's
Warfare Incorporated is one example that has multiplayer support. Just do a google search and you should be able to find more. Frankly speaking, I would suggest you invest in a dedicated portable gaming machine like the PSP if you are serious about handheld games.
As for applications for the WiFi hotspot or wireless network, the most obvious ones would be used to surf the Internet, check emails and instant messaging like MSN Messenger, Yahoo Messenger and etc. Other useful wireless apps include
VoIP calls,
IRC,
weather forecasts, file transfer via FTP,
online traffic monitoring... the list is unexhaustive. The most interesting one I've found so far is a
BitTorrent client for the PPC... ha!
Of course, you can operate all these with an ActiveSync passthrough (wired connection) but that will defeat the purpose of a mobile device, won't it? However, bear in mind that enabling WiFi on the PPC will drain the battery flat in just a few hours. Ultimately, it all depends on what you really need on your PPC...