Learn Italian: Questo software è pessimo, non ci voglio lavorare. - This software is awful, I don't want to work with it.

Here, ci means 'with it' (referring to the software). It acts as an instrumental clitic that replaces the phrase con questo software. This is a very common way to avoid repeating 'with something' or 'with someone'.

Could I attach ci to the end of the verb instead?

Yes! Because volere is a modal verb (a helping verb like want, can, or must), you have two choices for placement. You can put ci before the conjugated verb (non ci voglio lavorare) or attach it to the end of the infinitive by dropping the final -e (non voglio lavorarci).

I thought ci usually meant 'there' or 'us'. How can I tell when it means 'with it'?

You have to look at the verb and the context. The verb lavorare often takes the preposition con (to work with something or someone). Since ci can replace con + noun, and we are talking about the software, it naturally translates to 'I don't want to work with it'.

What is the difference between pessimo and peggio?

Pessimo means 'awful' or 'very bad' — it is the absolute superlative of cattivo (bad). Peggio and peggiore are comparative forms, meaning 'worse'. You use pessimo when something is completely terrible on its own, not just worse than something else.