Yes, there are several ways to do this.
- In CLIENTS > Email/Alerts there is a link labeled "Mass Email List". If you select some clients from the list and click this link, you will generate a list with three (tab-separated) columns: email address, first name, last name. If you use the "Filter Clients" link first, you can start with a shorter list; otherwise all clients are included in the list of names. You can copy and paste this simple list to a file editor, or you can paste it to a spreadsheet to remove columns and rows conveniently.
- In ADMIN > Reports there is a report labeled "Clients - Address Book". If you click this, you will get a spreadsheet with those columns and more. You can manipulate that sheet to strip out the rows and columns that do not interest you.
- In ADMIN > Reports there is a report labeled "Clients - Full Export". There's even more data about each client there.
- In ADMIN > Client List there is an "Export All Clients" link that works the same way as option 3. If you filter the clients first, you also get the option to "Export Filtered Clients".