Can HTTP POST be limitless?

EDIT (2019) This answer is now pretty redundant but there is another answer with more relevant information.

It rather depends on the web server and web browser:

Internet explorer All versions 2GB-1
Mozilla Firefox All versions 2GB-1
IIS 1-5 2GB-1
IIS 6 4GB-1

Although IIS only support 200KB by default, the metabase needs amending to increase this.

http://www.motobit.com/help/scptutl/pa98.htm

The POST method itself does not have any limit on the size of data.

Quite amazing how all answers talk about IIS, as if that were the only web server that mattered. Even back in 2010 when the question was asked, Apache had between 60% and 70% of the market share. Anyway,

• The HTTP protocol does not specify a limit.
• The POST method allows sending far more data than the GET method, which is limited by the URL length - about 2KB.
• The maximum POST request body size is configured on the HTTP server and typically ranges from
  1MB to 2GB
• The HTTP client (browser or other user agent) can have its own limitations. Therefore, the maximum POST body request size is min(serverMaximumSize, clientMaximumSize).

Here are the POST body sizes for some of the more popular HTTP servers:

• Nginx (largest web server market share as of April 2019) - default 1MB, no practical maximum (2**63)
• Apache - maximum 2GB, no default documented
• IIS - default 28.6MB for the request length, 2048 bytes for the query string; maximum undocumented
• InfluxDB - default ~25MB, maximum undocumented