- Incorporate a backdoor so you can access their site
- Put a hidden link to your site on their web pages
- Place an blatant link on their site without telling them
- Ask the owner if you can place a link on their site
Now the first one would be considered unethical, but the other three can help your site and reputation without hurting their site. Doing this for thousands of sites and you have got yourself some interesting links in.
Change niche to site building and you still have the same options and a niche where it's more acceptable. (Especially on templates)
So would you ask or would you just place it on there?