SugarSync gives a the moment (ends 15 July) 500 MB extra on top of their 2 GB free account if you use a referral link. Here is mine if you want to give it a try:
Yes, it is as with Dropbox even the one you refer gets 500 MB.
Unlike Dropbox during this time you can earn unlimited amount of bonus space
Yes, it is like Dropbox - backup, sync and share
Why use SugarSync instead of Dropbox?
I have no idea!
I have installed it to give it a try - easy as it's free
If you are using most of your free space at Dropbox and can't/will not upgrade and pay it can be a complement.
Offer with 500 MB extra on free accounts and 10 GB on paid is extended to 31 August
Edit: Removed the ref link - I got mine. Do use someone further down
Get DropBox - 2 GB + 250 MB for free