There are different options based on what you want to do.
If space is an issue (and you have an enough powerful computer to make conversions fast enough), you can convert all tour movies to AVI. An AVi is also a good solution if you watch movies on a computer screen, because an avi on a large TV screen can have an ugly quality.
Another solution is to rip all discs in main movie only format, i.e. saving in the HDD the main movie only. This can easily done with DVD Shrink or CloneDVD for example.
If you are interested in saving also extras then the only solution is to rip the full DVDs on the HDD.
One of the best softwares to play movies from HDD is VLC. It's free and it's available also in portable format (i.e. no installation required)