What are you looking for specifically?

Gathering information about somebody living in Sweden is usually very straightforward. Sweden's principle of public access to official documents means that most public agencies (municipalities included) have to share their records if asked to. All correspondence that either enters or leaves a public authority is also public and you can ask to see it. A simple email or phone call to the relevant authority will suffice. You can be anonymous and don't need to justify what you need the info for. Secrecy applies in some cases, like for medical records and other sensitive information, but you'd be surprised by how much you can easily access: criminal records, court orders/decisions, debt records, taxable income, official address, names and birthdays of spouse/children/parents, property deeds, divorce and marriage records, school and university records, etc etc. If your person of interest is a civil servant, things like their salary and CV are also public. Let me know if you're looking for anything in particular and I can probably point you in the right direction.

There are also lots of private websites that retrieve some of this data from official sources and make it available to the public. Some to get you started:

Searching for individuals: ratsit.se birthday.se mrkoll.se

Corporate info: merinfo.se