King Magnus Eriksson named Sigfrid as the patron saint of Sweden in the 1300s, and according to En svensk helgonkrönika, he is one of Sweden's more complicated saints mainly because there are so many discrepancies between what is historically known about him and the numerous legends surrounding his life, not to mention that there may have been two Sigfrids.
As missionary, Sigfrid was highly successful: He converted King Olof of Sweden (Olof Skötkonung) to Christianity by baptizing him in Husaby in around 1008 (at the time, most of Sweden was still pagan). St. Sigfrid's well near Husaby Church came to be known for its healing powers in the Middle Ages and became an important pilgrim destination.
After the conversion, while Sigfrid was away preaching, his three nephews (the Cluniac monks Winaman, Unaman and Sunaman) were beheaded by pagan raiders, notably Gunnar Gröpe. According to the legend, the heads of the three unfortunates were put into a weighted tub and thrown into Lake Helgasjön.
When Sigfrid returned and discovered the deed, he recovered the tub and claimed that the heads could still talk. King Olof offered to execute the murderers, but Sigfrid convinced him to spare their lives. The king then ordered them to pay blood money to Sigfrid, which he refused.
Sigfrid continued his mission and may have founded the parish of Växjö, Sweden, where he lived until his death on 15 February (the year is unknown). In 1342, King Magnus Eriksson granted Växjö a city charter "in the name of God and St. Sigfrid" ("Gudi till heder och Sancto Sigfrido").
St. Sigfrid is normally represented as one of three bishops on a ship, as baptizing King Olof, as a bishop menaced by devils or as a bishop carrying three severed heads (or loaves of bread, a misrepresentation of the heads).
Statue of St. Sigfrid, Övergran Church. Note the tub containing the heads of Winaman, Unaman and Sunaman.
During the Middle Ages, there were so many pilgrims who visited St. Sigfrid's grave in Växjö cathedral that Pope Clemens V ruled in 1352 that all who visited the church on the feast of the saint would receive a 140 day indulgence (which meant a temporary respite from Purgatory). Undertaking a pilgrimage to the church also gave an extra 100 days.
Sigfrid is still relevant today, mainly in the annual "Siffersmarknaden" (St. Sigfrid Market) held in Växjö each year on 15 February.
According to the Sibbo Swedish Parish's website (the Old Church in Sibbo, Finland is dedicated to St. Sigfrid, or "Sankte" as he is known there), a "Seffrasmessemarken" was even held prior to Christianity in Växjö on 15 February each year (though perhaps under a different name).
Cool fact about St. Sigfrid:
The Swedish pop band, The Ark, also hails from Växjö. They are mentioned together with St. Sigfrid, in the 2010 anthology of music in Växjö entitled Från Sigfridsmässa till The Ark (From the Feast of St. Sigfrid to The Ark.)