If you see this error either suddenly (no specific task was done to cause the error) or frequently, try deactivating all plugins to rule-out a plugin-specific issue and try switching themes to rule-out a theme-specific issue.
Specific fixes:
1. If you’re using WordPress 2.9.2 or lower, try adding define(‘WP_MEMORY_LIMIT’, ‘256M’); to your wp-config.php file. If you’re using WordPress 3.0 or higher, WordPress automatically does this for a variety of tasks, so there’s really no reason to try it in this case.
2. If you can edit or override the system php.ini file, increase the memory limit. For example, memory_limit = 256M
3. If you cannot edit or override the system php.ini file, add php_value memory_limit 256M to your .htaccess file.