I've been accumulating this list for a few years, but never published it. Not sure why.
Anyway, here's some free stuff you can get in Boulder on your birthday. Most of it requires some planning in advance. I won't be in Boulder for my birthday this year (I'll be here in Philly, at the dentist's. Really. Wooooooooo.) So hopefully this list will be of more use to you.
Aveda - Coupon for free birthday gift (usually a fullsize product) redeemable at Aveda salons. Sign up online.
BJ's Brewhouse - Free Pizookie dessert. Email signup.
Cork - Free entree. Join their Connoisseur Dining Club.
Dairy Queen - Join their Blizzard fan Club, receive a coupon for a free birthday Blizzard
Daphne's Greek Cafe - Free entree. Email signup.
Denny's - Free Grand Slam, just show up with your driver's license.
Firehouse Subs - show your ID, get a free sub.
Glacier Ice Cream - Free ice cream cone OR small gelato on your birthday. Join their Birthday Club online. (7 days in advance)
Noodles & Co - Sign up for their email newsletter to receive a coupon for a free birthday meal. (60 days prior)
Red Robin - Join their e-club, get a free birthday burger.
The Sink - Free appetizer or cookie on your birthday. Sign up for their S Card program.
Spicy Pickle - Sign up for the email list, get a coupon for a free birthday sandwich. (You might have to buy a drink.)
Starbucks - Register any giftcard you have for them online, a free birthday drink should come up when you check your rewards.
Ted's Montana Grill - Free dessert. Email signup.
Tee and Cakes - Join their Birthday Club. $5 gift certificate in the mail. Must sign up prior to birthday month.
The Bus Stop - Free lap dances.
If you're willing to trek to Denver:
The Broker - Buy one entree, get one free. Plan it in advance w/ them, and bring a friend.
Denver Zoo: Free admission on your birthday. Register online in advance.
The Fresh Fish Company - Not free, but members of their birthday club receive a percentage discount equal to their age. Sign up online.
LaMar's - free donut & coffee when you show up in person & join their LaMar's for life program. Register your card online & they'll load a free birthday donut on it.
That's all I've dug up. If there's a local deal I've missed, let me know!