Each site I have reviewed give you their own little something to say 'thank you for being a customer' each and every time you top up your account with them.

Here is a run down of who's giving what and what the best deals are:

MP3va 
This site offers free credit when you top up:

Top up $29.95 and they give you $15.00
Top up $49.95 and they give you $50.00

MP3Million 
This has site has two deals running:

When you top up €29.95, they give you €10.00
When you top up €49.95, they give you €50.00
Or:
When you top up €19.95, they give you 500 Free Downloads for a month
When you top up €49.95, they give you 1500 Free Downloads for a month

You chose which deal you want when you go into the 'add to balance' screen. I've written about how to get each deal in full on a previous post: MP3 Million Top Up Offers