In online life, the most annoying thing is probably spam. Followed by pop-ups/unders, and so forth, and sites that are almost entirely a new version of Flash that doesn’t work with Linux.
Right after that, the forums that turn up in Google searches and require that you have an account to actually read them. Yeah, because I want to have to register for about ten new sites a week just to read one damn post.
I don’t mind (that much) sites that require a valid account to post but I despise having to sign up for an account off of dozens or hundreds of sites just to read one bloody article.
For logins to read forums or webpages, check BugMeNot! and it’s Roachfiend firefox extention.
When a site requires a valid email account using Mailinator.com is a spam saver. Btw mailinator.com is being banned from a lot of website, but not their alternate domain names (fakeinformation.com, sogetthis.com, and mailinater.com). It’s not secure, but in most cases it’s doesn’t matter (downloading a file for example). There’s a lot of those around try to google for such services.
Btw, it’s the topic just now on Digg.com.