Andréa Coutu wrote recently about 7 terrible secrets revealed by your email address (and how to fix them). Her advice is good, and I recommend reviewing each of her points before you decide on your "official" email address.
If you're in business as an IT consultant, you have no excuse for not having your own domain name. If domain registration and hosting breaks your bank, then you shouldn't be in this business. Even if your niche has nothing to do with the Internet (as unlikely as that's becoming), nothing says "I'm not really serious" like an account on hotmail.com, yahoo.com, especially aol.com, and even gmail.com (unless you're a Google employee). Not that you can't also have one of those addresses (except aol.com, what were you thinking?!), but don't use it professionally.
When choosing your domain name, you should use the name of your business. If your business doesn't have a name, get one. Even if it's just "your name here Consulting," you should present yourself as a commercial entity. If you feel that your business name is too long for a domain name, you should make sure your abbreviation seems natural and obvious. For example, my business name is "Camden Software Consulting," and my domain name is "camdensoftware.com". If I had chosen something like "camdenswcnsltng.com," then my contacts would always have to look it up to remember how I abbreviated it. Also watch out for unintended words that arise from combining abbreviations. You wouldn't want to abbreviate "Megara Associates, Inc." as "megastinc.com" for example.
I've seen some independents who treat their domain name like an 800 number: they make it into an ad. Domains like "peoriacomputerwiz.com" may be cute, but unless it's also the name of your business, your client will have one more thing to remember when they want to contact you. "Was it peoriacomputerguy.com, or peoriapcguy.com? Or wait, isn't their office in Pekin?"
For the top-level domain (TLD), I think ".com" is preferable. It means "commercial" (you're in business here, aren't you?) and despite being US in origin, it has international applicability. It's also what flies off people's fingers automatically when they're typing a domain. If you limit your business to one country or region, then a nation-specific commercial TLD could also be appropriate. The ".org" TLD says "I'm a non-profit!" even though you don't have to use it for that. The ".net" TLD is a little better, but people tend to infer some sort of online community instead of a business. You should avoid ".biz" and ".info" — the spammers polluted that space years ago, and your emails will get filtered for that reason alone.
There's nothing wrong with registering the same domain in several TLDs and redirecting them all to the same address, though. In fact, it's a good way to keep other people from using your business name.
For the local part of your email address (the part preceding the "@"), I suggest just using your name. I don't care if it's your first name, your last name, or first.last name — as long as it's your name. An Internet pseudonym comes across as unprofessional here. On the other end of the spectrum, "sales@" or "info@" seem too impersonal. Sure, you can establish those addresses to catch the wayward or the cold contacts, but when you give a customer your email address, it's a good idea to make it like a handshake.
So, how does your email address compare with these criteria?
Chip Camden has been programming since 1978, and he's still not done. An independent consultant since 1991, Chip specializes in software development tools, languages, and migration to new technology. Besides writing for TechRepublic's IT Consultant blog, he also contributes to [Geeks Are Sexy] Technology News and his two personal blogs, Chip's Quips and Chip's Tips for Developers.