I accidentally deleted a wrong class via the Force IDE and want to restore / undelete it. I found no way to do this....
There is an option to Restore from Local History in Eclipse. You could give that a go.
This isn't directly an answer, but whenever something like this happens it's a great time to start looking at source control. While I've always felt that Salesforce should be versioning these things for us, that doesn't appear to be likely anytime in the near future.
A simple solution we've used is just to do a daily checkout of all of our apex code and push it to a repo. It lacks much insight into why those changes were made, but does save us from accidental deletions and other common mishaps.
For a more full-fledged solution, take a look at StratoSource RedHat's internal Salesforce project management tool which includes auto-repos and other powerful goodness to manage your internal Salesforce projects.
I did get this technique (Bascially, querying for the deleted class in Dev Console, and then using the Id to display class which seems to work even when class was deleted) to work for me once, but trying it in a dev org now didn't seem to work - but wanted to throw it out in there in case
They way I do it is to use the execute anonymous function from the Developer Console and run the following:
system.debug([SELECT Id FROM ApexClass WHERE Name = 'NAME' all rows]);
Then open the class via https://instance.salesforce.com/Id click edit then save and voila the class is back.