ISO-8859-1 vs UTF-8?

Unicode is taking over and has already surpassed all others. I suggest you hop on the train right now.

Note that there are several flavors of unicode. Joel Spolsky gives an overview.

(Graph current as of Feb. 2012, see comment below for more exact values.)

UTF-8 is supported everywhere on the web. Only in specific applications is it not. You should always use UTF-8 if you can.

The downside is that for languages such as Chinese, UTF-8 takes more space than, say, UTF-16. But if you don't plan on going Chinese, or even if you do go Chinese then UTF-8 is fine.

The only cons against using UTF-8 is that it takes more space compared to various encodings, but compared to western languages it takes almost no extra space at all, except for very special characters, and those extra bytes you can live with. We are in 2009 after all. ;)