Another vote for Basecamp. Been using it now for 2 years and and, critically, has very high client adoption rates (90%+) which massively simplifies all our lives. There is a even a free version to get you going.

It is my understanding that basecamp requires you to purchase mutiple apps per month. Are there any affordable fully integrated project management web apps that do billing and crm too? <!--table {mso-displayed-decimal-separator:"\."; mso-displayed-thousand-separator:"\,";} .font5 {color:windowtext; font-size:8.0pt; font-weight:400; font-style:normal; text-decoration:none; font-family:Arial; mso-generic-font-family:auto; mso-font-charset:0;} td {padding:0px; mso-ignore:padding; color:windowtext; font-size:10.0pt; font-weight:400; font-style:normal; text-decoration:none; font-family:Arial; mso-generic-font-family:auto; mso-font-charset:0; mso-number-format:General; text-align:general; vertical-align:bottom; border:none; mso-background-source:auto; mso-pattern:auto; mso-protection:locked visible; white-space:nowrap; mso-rotate:0;} .xl65 {white-space:normal;} ruby {ruby-align:left;} rt {color:windowtext; font-size:8.0pt; font-weight:400; font-style:normal; text-decoration:none; font-family:Arial; mso-generic-font-family:auto; mso-font-charset:0; mso-char-type:none; display:none;} -->

Oops. Hit enter too quickly. If you look at the 37Signals site it will show the apps that work with BC. Fully integrated is only any good if the whole app is good; using API stuff allows you to mix and match and pick the best bits that best fit you business.