How open is my ip, & what shoud i do?

[Macoy]

If i go to a site, how can i be easily anonymous?

[Orangutan]

Depends,

Do you want to hide who you are from the site or where you go from your ISP?

Edit I think it's the former.

start here:
http://en.wikipedia.org/wiki/Anonymous_web_browsing

[Macoy]

Just for example: i want to visit a site like gosylvia.com and leave a message without the webmaster publishing the address of my isp

[Orangutan]

You could configure your web browser to use a public proxy server, here is a start,
http://www.publicproxyservers.com/
You need to find the proxy server settings in your browser though.
In firefox it's tools -> options -> advanced -> network -> Connection - Settings.

or you could try a site like http://anonymouse.org/ which claims anonimity to your surfing, but if scrips are run on the page I don't know how anonymouse it is.

Perhaps someone else knows of a better choice. Of course, the anonimising service will have you IP but not the destination.

[Macoy]

Probably a bit too furtive & paranoia inducing. Still is annoying that that particular site shows isps.

[bruto]

Assuming you didn't want to do this habitually, how about just going down to the public library or some place like that, and logging in from there? If you wanted to make a habit of it, you could get a laptop with wifi and connect from coffee shops, libraries, laundromats, or whatever. You can do a lot of mischief in the time it takes to drink a couple of cups of Panera coffee.