First of all: commenting is open to everyone, anonymous or not.
If a giveaway is hosted here there are a couple of things you might like to know:
Dreamwidth Users:This should be fairly straightforward. If you have a DW account and comment on an entry here, I (or your giveaway host du jour) will have a way to contact you, either through your account email or via a comment on your DW.
OpenID UsersWhat's OpenID, you ask? Well, it's a distributed identity system, meaning you can use one login across many websites.
Chances are pretty good that you already have an OpenID, for example through your LiveJournal account [
username.livejournal.com would then be your OpenID], or Flickr, or Google, or Yahoo, etc. pp.
This site should be quite helpful in that regard.
Dreamwidth lets you set up an OpenID, which is awesome because a) you can upload up to six icons b) you're more easily identifiable without having to resort to IP addresses, and c) you have some control over your comments, such as deleting them.
I encourage you to use your LJ account should you have one.
Anonymous CommentersIf you are none of the above or don't want to bother (trust me, I understand), then
providing an email address or another easy and reasonably secure way to contact you is - well not crucially important, but close.
IP tracking is on, so I know who you are, but unless you check back manually you won't know that I've answered your comment and I don't have any other way to contact you.
And finally ....If you would like to have a Dreamwidth account, tell me and I will send you an invite (preferably to your valid email address). :)