Your site could have the best content and the most beautifull images, but when it takes a long time to load your pages, or when the contrast between the color of your text and background is low, your visitors will leave and never come back.
Luckily there are a lot of tools to analise and optimise sites.
gtmetrix measures your page speed. This free to use on-line tool uses 02 different instruments Google's Page Speed and Yahoo's YSlow and offers recomendations to increase the performance.
Another tool is Webpagetest with whom you can indicate the test place (e.g. São Paulo or Buenos Aires for South America) and the browser (e.g. IE8, IE9, Chrome or Firefox).
Measure the velocity of your internet connection
When you load a webpage, your computer and the server that hosts the site, will need some time to process their tasks. The server has to create the page, your computer has to download and render it. With tools like http://www.minhaconexao.com.br/ you can measer the velocity of your internet connection. A high speed means less time to visualise the pages.
Avaliation of accessibility
The Ministery of ério do Planning, Budget and Management (Brasil) developped e-MAG or Modelo de Acessibilidade de Governo Eletrônico (only in portuguese). Unfortunately, the avaliation part ASES is in a process of actualisation.
In the mean time you can use AChecker.
Contrast of colors
CheckMyColours is an online tool to test combinations of text and background colors. It uses algorithms suggested by the World Wide Web Consortium (W3C).