If you need links for any of these, let me know.

General - Adding reader comments to your blog On Blog*Spot, or off if you don't have server-side scripting (PHP, ASP, CGI):

Simple to install, but not much control over your comments:
* BlogBack - not currently accepting new signups.
* YACCS - not currently accepting new signups.
* netcomments - currently down while searching for new hosting.
* enetation
* haloscan

Not so simple, but you control your own comments:
* Remotely hosted dotcomments

Not on Blog*Spot, with server-side scripting

Got PHP?:
* dotcomments
* Poster Child
* blogKomm Comments are displayed in the page below each post, where everyone will read them, instead of in a popup, where only people who have already decided to comment will read them.
* Tara Star's version of Sascha's script
* BlogBack PHP A GPL'ed version of Marcus' original BlogBack script, suitable for hosting comments for ten or twenty blogs.

Got ASP?:
* XComments (no longer supported - link goes directly to zip file)
* aspcomments - ASP/Access db
* Blogcomments - ASP/Access db

Got CGI?:
* cgiComments
* snorcomments2 - a locally-hosted version of the former remotely-hosted snorcomments, which lets you create up to 99 separate comment accounts, so that you can also host comments for 98 of your closest friends. Features include your choice of a quick, low-bandwidth version without comment counts or a slightly fatter version with counts, comment editing and deleting, and templates. The templates are controlled by the comment host, not by users, so if you host your friends they will have to send their templates to you, but you can include whatever you want (say, a link to your blog, or a text ad...) in their comment window. If this catches on, it will be a great way to ease the strain on remotely-hosted systems.