I've always wanted to improve the climbing ability of my Bike Friday tikit - and the cheapest way is to reduce the size of chain ring from 53t to 42t. I managed to find an unused 1980s new Shimano Biopace elliptical 42t chain ring at Rebound Centre which was duly installed on my tikit.
Effectively reducing the gearing by 20%, the tikit now can really tackle those hills easy. Downside is the downhill performance isn't so good as you tend to spin out faster - after 35km/h or so. Such are the things in life I suppose.
More info about the advantages of elliptical chain rings here by the late guru, Sheldon Brown.
Best $30 zhnged mod ever, I reckon...