[lobster_johnson]

I use this one (supports Chrome and Safari):

http://hckrnews.com/about.html#extensions

It has collapsing, and it also highlights unread comments.

[aganders3]

Thanks - much appreciated as a Safari user.

[peter_l_downs]

Just installed it – thanks for the pointer. Works exactly as expected.

[dionidium]

This is the one I use, too. It does exactly what you need it to do.

[kawera]

Excellent extension, been using it for years.