I memorised a very simple algorithm to construct passwords from the domain name of a site. Then I concatenate that with one of three fixed portions depending on how important I view the site (e.g. banks get the most secure one, then gmail, then everything else).

It works pretty well. Different password for each site, I only have to remember a few things, and it would take several compromises (and a weirdly dedicated attacker) to work out my algorithm.

For example, my 'insecure' fixed part might be 'Tenk5$' (I recommend including uppercase, lowercase, a number and a symbol in that part to get around idiotic password requirements). Then my algorithm could be 'the last 5 letters backwards, skip the first vowel'. In which case my password for HN would be 'Tenk5$rtani'.