All birthday freebies require signing up to their respective mailing list with an email. The sister-in-law goes all crazy signing up for these lists and gets all sorts of free food around her birthday. They usually send you news on new items, promos, and coupons as well. I suggest just creating an email specifically to sign up so you don't get inundated with promo emails. (Work in Progress... Last updated: July 13, 2012)
As to why companies would give your free stuff for your birthday, it's for a number of reasons including, it builds good will (i.e. you might come back), if it's your birthday, you'll usually come with other guests, and you might order other items while you're there.
Please do keep in mind that you'll need to print out a coupon and they may check your ID. Most coupons are valid for a least a two week period around your birthday. Also, they can change the offers at any time.
Acapulco - Fiesta Amigo - Free guacamole for signing up and free entrée on your birthday.
Baskin Robbins - Birthday Club - Free 2.5oz. scoop on your birthday. Emails you a coupon 5 days before your birthday.
Ben & Jerry's - Be a Chunk Spelunker - Free Scoop for signing up and another free scoop for your birthday.
Benihana - The Chef's Table - Free Meal up to $30 (excludes alcoholic beverages, gratuity and tax) - you need to sign up at least 10 days before your birthday to make sure to get the certificate in time. They send it at the beginning of the month of your birthday and it's good for that entire month.
Black Angus - Prime Club - Receive a complimentary dessert for joining the Prime Club and a free steak dinner on your 1st birthday as a Prime Club member - the free steak dinner requires the purchase of a second entree of greater or equal value and is good for 2 week before and after your birthday.
Claim Jumper - E-Club - Coupon for Free Entree on your birthday
Cold Stone Creamery - Birthday Club - printable coupon for free ice cream on your birthday - "Your birthday coupon will be available in your My Cold Stone account seven (7) days prior to your birthday. It will be available to print from the My Birthday Club page in your account for 14 days, seven (7) days before and seven (7) days after your birthday."
Chili's - Free Dessert on your birthday
Daphne's Greek Cafe - Pita Points E-Club - Frequent customer program and Free meal on your birthday.
Del Taco - Raving Fan E-Club - 2 Free Chicken Soft Taco to sign up and free Premium Shake on your birthday.
Dunkin' Donuts - DD Perks - new members get a mailed coupon for a free medium beverage and all members get a coupon for a free medium beverage during the month of their birthday. - current members do have to update their birthday info to take advantage.
El Torito - Bull's Ring - Free Guacamole now, Free Entree on your birthday
Firehouse Subs - Birthday Club - Free Sub on your birthday
Fuddruckers - Fudds Club - Free Gift for Joining and Free Burger for your birthday.
Hooters - 10 Free Wings on your birthday
Houlihan's - Email Club - Free App for Joining and Free Entree on your birthday
IHOP - Pancake Revolution - Free Meal for Signing Up and on your 1-year anniversary of joining
Jersey Mike's - Email Club - Free Regular Sub & Drink Coupon on your birthday.
Lee's Famous Recipe Chicken - Free meal on your birthday.
Logan's Roadhouse - Free Dessert on your birthday.
Moe's Southwest Grill - Free entree of your choice (excluding fajitas) on your birthday.
Panera Bread - Free Baked Good on your birthday
Red Robin - eClub - Free Burger (comes with Bottomless Fries) on your birthday
Rubio's Fresh Mexican Grill - Beach Club - Free Taco for signing up and Free Food up to $7 on your birthday - That's a lot of Fish Tacos on a Taco Tuesdays.
Starbucks - My Starbucks Rewards - Free "handcrafted birthday beverage" of your choice on your birthday and frequent buyer rewards.
TCBY - eClub - free waffle cone for signing up and unspecified "special gifts" on your birthday.
Tony Roma's - Roma Ribs Expert Club - Free Entree on your birthday