I only found this out the other week having apparently been totally out of the loop: the official YouTube channel for Power Rangers has been uploading the entire series, full episodes, upscaled in HD.

As of now, they're on the fourth overall season aka Zeo. Uploaded in full thus far:

• Mighty Morphin' S1, S2, S3

• Mighty Morphin' Alien Rangers (Part of S3)

• Wild Force S10

• Dino Thunder S12

• Jungle Fury S16

• Dino Charge & Dino Super Charge

I largely stopped watching the show around 2003/2004 - the Ninja Storm / Dino Thunder seasons - but it's awesome to be able to go back the classic seasons from the beginning like this.