We do PRLog dot org and it helps us a lot. (We usually go after long tail KWs). Depending on the subject - we usually get a fair amount of referral traffic from there - even months later we'll get some here and there.
From this list, I use prlog.org and free-press-release.com. Both approve the PRs instantly.
I like prlog more. Urls in the body of the text are clickable. Images are allowed in the free version. Btw, make sure you include .jpg images only. I was attaching .png's for quite a while wondering why they don't show up in search results. Only .jpg's do.
Thanks for suggesting the three other, I'll try them out with my next submission campaign.
Here are some more well-tested suggestions for you:
