UVa Online Judge is a problem archive with over 3500 problems.
Start off by reading the the introduction.
Are there any problems that you would like us to do? You can request problems through the contact form.
A list of the solutions available on this site:
Problem Set Volumes
100 – The 3n + 1 problem
102 – Ecological bin packing
294 – Divisors
498 – Polly the polynomial
530 – Binomial showdown
10055 – Hashmat the brave warrior
10127 – Ones
10229 – Modular fibonacci
10268 – 498-bis
10783 – Odd sum