Are there any tools that would let multiple people edit things like sprites or code at the same time? Perhaps something that works with services that host code.
Try GitHub for version control. It's fantastic -- you can use git or svn (whichever you're more comfortable with) and your repositories get free issue tracking, wikis and cool visual graphs. The only downside is that if you want your project hosted privately, you have to pay. (It's as low as $7/month as of this writing; we pay and find it well worth the cost.)
We also love and use Dropbox. It supports Windows, Mac and Linux and features automatic file syncing across multiple machines. You can even hook it into Growl to see what your collaborators are doing realtime! Best of all, the basic plan is free and gives you a very generous 2GB of storage. (Note: should be mentioned to be careful with Dropbox files, as your collaborators could inadvertently destroy your changes. Luckily Dropbox provides some versioning but extra precautions on your end are recommended!)
Google Docs is one way you can share and edit design documents.