dotComics is a program that Marvel Comics has made available on their website.
It allows you to view issues from various series produced by them for absolutely free.
All that is required is that you give them your email address (aka sell your sole to advertisements) and download the program. You can then update it periodically, via the useful Update button, and see what new comics are available for download.
Each comic takes up about a meg and a half. The comics each come in two parts, each part having 16 pages of the comic.
The auto-pop feature is nice, as you don't have to click on each panel to view them, you just click and it automatically scrolls to the next panel. The only downside is when it hits an advertisement, you have to wait for it to be done before continuing.
Overall, it is a very useful program. Especially if you've missed some key issues. On dotComics, you can find Origin #1 which tells of Wolverine's origins, as well as the X-men issue where they cure the Legacy Virus.
I highly recommend this program to comic fans who are having trouble finding certain issues, or lack the funds to buy them. Though it currently only has around 70 files on it, this number will grow with time.