It has been a while since I have posted a list of recommended add-ons for X-Cart, and in that time some great ones have come out. First, there are a few that are essential IMHO:

CDSEO - this add-on gives you complete control over your URLs, META data, Title tags and links. This is a mod you simply cannot be without if you expect to do well in the search engines. Well worth the price, and the support is excellent. DO NOT buy the XCSEO alternative that has been gorilla-marketed in the forums. Not only will you receive zero support by the company who sells it, chances are it will completely disable your site when you upgrade your site.

On Sale - If you want to run any sort of promotion on your site from free shipping to offering free products, this is the mod you need. Also excellent support from this company.

Smart Search - if you have a product catalog of over 100 products, it is important to have a good site search tool. The X-Cart default search is awful - this mod works much like Google in that it can handle misspellings, plural/singular, etc

One Page Checkout - the key to conversions is to make the checkout process as easy and simple as possible. If you are one of our clients you have our streamlined checkout to do this for you, but if you aren't, this is a great alternative. This mod uses Ajax to put everything related to the checkout on one page. The customer will have paid for the order before they even know what hits them!

The next group are great mods as well if you are looking for particular functionality:

Buy Together - another great mod from Altered Cart, this one lets you set up 'deals' when you buy two (or more) products together like Amazon.com does. Also automatically recommends products that 'customers also bought' which saves you hours of manually adding upselling links.

Marketing Manager Pro - this is a great tool to help you better understand how customers are using your store, and it also gives you some great reporting abilities (something severely lacking in X-Cart).

T-Hub - if you are looking for a solution to seamlessly integrate X-Cart, Quickbooks and all of the major shippers (UPS, USPS, FedEx) this is the one!

Flyout Menus - if you are looking for a dynamic category menu, Phil has many choices available.

Customer Review Management - by default, X-Cart doesn't have much control over product reviews. This tool provides the missing functionality that SHOULD be there already.

Remember Anonymous Carts - I'm sure you all do it - you go to a store to browse, add a few things to your cart and come back later to buy. In X-Cart, unless you are logged in, that cart is lost as soon as you close the browser. This add-on saves the cart, even if you aren't logged in.

Power Filtering - another feature missing in X-Cart is the ability to 'filter' the product display within a category. This mod lets you define those filters in the admin so that your customers can easily find what they are looking for.