Sometimes chords can be substituted for others, as long as they remain in the same mode (major, minor or dominant). Usually (but not always) these chords are just extensions of the triad.
The I chord is major, and is commonly replaced by a major 7, major 6 or major 9 chord, especially when you have several bars of the same chord and want to make it sound more exciting.
In blues music, a dominant 7 is also sometimes used instead of the I chord. Additionally, the III chord (which is minor) is also a common substitution for the I chord.
The II and III chords are minor, and can be replaced by a minor 7, minor 6 or minor 9 chord.
The IV chord is major and is commonly replaced by a dominant 7th chord in blues. We can also substitute a major 7 or major 6 similar to the I chord.
Another common trick on the IV chord is to play the major chord and then follow it with a minor chord of the same root, or to replace the major IV chord with it's parallel minor chord altogether. This occurs in songs such as 'My Way' by Frank Sinatra and 'Nobody Home' by Pink Floyd (see Modal Interchange).
The V chord is dominant, and as such any number of extended and/or altered chords can be substituted.
We can use a 7th, 9th, 11th, 13th, 7b9, 7#9, 7b5, 13b5b9, 11#5, 7#5b9 - the list is extensive. It really depends on the progression that it fits into.
It is also possible to substitute a diminished chord for the V chord. A lot of popular songs, however, simply use the dominant 7 as a substitution for the V chord.
We can also use a minor chord to replace or precede the dominant chord. This minor chord has a root note a fifth higher than the dominant chord, and it is a close relative of the dominant. If preceding the V chord, this additional chord then creates a brief II-V-I progression in the music.