If you have a domain you do not want any more, try to find an organization to donate it to, rather than letting it expire.
There are unscrupulous Domain Vultures, who scan lists of domain names that are about to expire. Just like vultures, they feed off the deceased. They buy them for about $6 per domain, then post pornography or something so disgusting that the original owner will want to buy them back.
Domain Vultures realize that domains had accumulated a lot of hits on search engines, and that there may still be some association of the original owner with the domain. They may then "reluctantly" agree to sell the domains for many times their investment, such as $400.
Domain Vultures also realize that some domains expire unintentionally, because the owner forgot, or missed the renewal reminder email. They then can exact fees of many thousands of dollars, if you really needed your domain, and its associated emails.
Remember also that for many months, perhaps years, the email for your expiring domain, will go to the Domain Vulture.
Do not allow them to stay in business, or make a profit at this. Renew your domains with http://CIReg.com for under $9 per year, or transfer it to an organization of your choice that would like the domain.
Especially do not allow the domains you want to expire. You can best do that by putting them on AutoRenewal - so that the domain registration system will automatically renew them, and charge your credit card the small annual fee fee.