Post blog entries and get them corrected by native speakers:
http://lang-8.com/ - I have the most experience with this one. I find it simple and straightforward to use. Help other people while they help you.
http://livemocha.com/ - I believe this is similar to lang-8 but have not used it much.
http://www.interpals.net/ - This is like MySpace for language learning. You can use it to practice languages seriously, or you can just chat with people for fun.