Market, LauncherPro, My Paper Plane2.... that is all
I would suggest going with at least a class4 micro sdhc's. I have 2 16GB class4 cards and they run video and audio just fine. Sometimes takes a couple seconds to buffer up but after that no problems. Look around on the web though as you can usually find some great deals. I got these for $20 each and I heard that you can find 16GB class 10's for around $30. Don't bother with the 32GB's yet as they are overpriced for now at around $100. Also watch were you get them from, there are a bunch of fake ones floating around on ebay right now, stick to a more reliable place to purchase them.
Wifi problems? Reboot router first. This has worked everytime since the 0.71 firmware update. (for me anyway, YMMV)