Cookie cutters make awesome templates for onesie designs because they are just the right size and not so detailed that they are impossible to sew around. I got my Missouri cookie cutter at Off The Beaten Path. They have some adorable tool and train cookie cutters that I've been eyeing - great for a boy's shirt. 
I drew the patterns for the guitar, whale, and tie myself. If you are not confident in your sketching abilities, do a search around the web for free templates. Here are a few to get you started:- Find a different tie and guitar over at Trey and Lucy
- Get a squirrel template at the long thread
- Make a simple cupcake at bored and crafty
- Jordan Ferney just posted a tutorial at Cookie with a link a great picture font (perfect for templates)
- You can count on Martha to be a reliable resource for cute templates, (especially animal templates) - I like her decoupage animals, bib appliques, animal silhouettes, bunnies, and bulldog. And I really love the owl and elephant templates from petite collage that posted online recently from the tv show.
Although I don't have any photos of the ones I've personally made, just plain initials are always a big hit. You can find a tutorial for it at this mama makes stuff. Numbers work well too, especially for birthdays.
Of course, with the right fabric print, no template is required. Just cut out a circle or square piece and sew. Here is another example at armelle blog.If you are feeling really adventurous, you could try out reverse applique.
