A nice website that offers timed practice for adding, subtracting multiplying and dividing. You can choose which operations to practice and how large the numbers will be. The nice thing about this website is that you don't have to hit "ENTER" after you answer. If your answer is right you get another question - if it's wrong nothing happens until you enter the right answer.
I recommend Level 2 problems for mental practice. At that level the sums and differences tend to stay below 25. One good thing about this site is that it will apparently generate an unlimited number of problems for you to practice.
This web-site doesn't offer mixed practice - just addition or multiplication. You get 40 problems and the computer times how long it takes you to complete them. The links for the two pages are below: