You can NOT play on servers with online mode set to true(“official” servers)
But if they don’t have that it should work just fine, just make sure you don’t use the same nick as an existing user because that will stop you from logging in :)
Please consider buying the game(for it is awesome) at minecraft.net, it’s only 20€ and totally worth it!
Useful links for minecraft:
Bukkit is for 1.0.0 now, so get ready for a lot of modded servers!